1. Field of the Invention
The invention relates to data processing systems in general and more particularly to the detection of a transmit underrun state for generating an abort sequence of bits over a communication line in a communications subsystem.
2. Description of the Prior Art
Highly flexible and cost effective communications subsystems have been provided for coupling data processing systems to communication channels such as those associated with a publicly accessible (e.g. telephone) communications network. Such systems have been embodied in hardware/firmware architectures which respond to commands from a communication processor associated with a central processing unit and enter into either a receive mode, a transmit mode, or concurrent transmit/receive modes for transferring data messages between the communication processor and the communication channel. Data transfers occur under a control element acting in concert with a microprocessor within the adapter to assemble and disassemble whole or partial data bytes of varying bit sizes. System architectures which have been used readily accommodate an expansion of capacity and exhibit dynamic flexibility.
However, present communication line adapter systems are not able to provide the control functions required for interfacing with a synchronous communications network such as, for example, has been specified by the International Telegraph and Telephone Consultative Committee (CCITT) and described in the "CCITT Sixth Plenary Assembly-Orange Book, Vol. VIII.2 Public Data Networks", published in 1977 by the International Telecommunication Union, Geneva. Various specifications, in particular those designated X.21, X.24 and X.27, promulgated by the CCITT call for generation within the data network of bit and byte timing signals which are used for establishing a byte control protocol (BCP) or a bit oriented protocol (BOP) for organizing and handling data messages.
In a communication environment such as X.21, it is necessary to use different types of protocols (i.e., byte control protocol and bit oriented protocol) in order to send and/or receive messages with other data communication equipment. During the bit oriented protocol transmission mode, the communications subsystem must receive sufficient data to maintain a continuous stream of data bits over the communication line. If for some reason sufficient data is not provided, then a transmit underrun state is sensed and a series of binary ONE bits indicating an abort is sent over the communication line.
In the Honeywell Level 6/30 System, a microprocessor was designed to send an abort sequence. The microprocessor detected the transmit underrun condition by logic not responding to a data service request in sufficient time to maintain continuous data transmission. The microprocessor generated a single abort sequence.
The microprocessor approach required considerable logic and time consuming firmware and software routines to perform all of the required data communication function.
U.S. patent application Ser. No. 053,111, which was abandoned May 29, 1981 entitled, "Communication Line Adapter for a Bit and Byte Synchronized Data Network" and U.S. Pat. No. 4,254,462 issued Mar. 3, 1981 entitled, "Hardware/Firmware Communication Line Adapter" having the same assignee as the instant application describe such a system whereby the transmit underrun state is processed primarily with firmware/software routines. This generated the problem of restricting the data throughput of the system.
Replacing the microprocessor and its associated logic with the Signetics Multi-Protocol Communications 2652 Circuit reduced the required hardware significantly. However, the use of the 2652 circuit presented problems in several areas of generating an abort sequence in response to a transmit underrun condition. A principle problem was the possibility of sending 16 or more successive binary ONE bits out on the communication line. The receiving device sensed the 16 successive binary ONE bits as indicating that the line was in an idle link state rather than in a transmit underrun state.
It should be understood that the references cited herein are those which the Applicants are aware of, and are presented to acquaint the reader with the level of skill in the art and may not be the closest reference to the invention. No representation is made that any search has been conducted by Applicants.